A fundamental property of polymer melts is their non-Newtonian viscosity, i.e., as applied shear rate increases, the shear viscosity decreases. The underlying molecular cause of this effect is molecular orientation in the direction of flow. A measurement of molecular orientation at a given value of shear rate can be used to deduce the value of the non-Newtonian viscosity. Optical parameters which can be used to monitor molecular orientation are birefringence, dichroism, and fluorescence anisotropy. The first two measurements require the transmission of light through the subject polymer material, but fluorescence anisotropy can be measured via excitation and detection from the same surface. Because of this, fluorescence measurements can be carried out on opaque or translucent filled polymers by examining the "front" surface. A single probe, used to excite and collect fluorescence light, is sufficient for the fluorescence observations.
If a polymer contains fluorescent moieties within its structure, then fluorescence anisotropy measurements can be made without adding a tracer dye. However, most polymer materials are not inherently fluorescent and the incorporation of a dye is necessary in order to use fluorescence to monitor the properties of interest. For application of fluorescence monitoring, the dye must have a molecular weight or aspect ratio high enough so that it can reflect the polymer-like behavior of the polymer matrix such as molecular orientation under the influence of an applied shear and/or extensional stresses.
The use of fluorescence measurements to monitor the molecular orientation is the subject of U.S. Pat. No. 4,521,111 to C. M. Paulson and M. E. Faulhaber. Their technique consists of monitoring the fluorescence intensity as the excitation polarizer is continuously rotated. When the direction of excitation light is coincident with the average direction of the fluorescence absorption vector, maximum fluorescence intensity is observed. When examining an oriented specimen, a sinusoidally alternating intensity is generated by this technique. However, this method does not yield values of the fluorescence anisotropy and moments of the distribution. Also, Paulson and Faulhaber do not attempt to measure non-Newtonian viscosity.
Fluorescence has been used to measure the molecular orientation in solid polymer films (Nobbs et al., Polymer, 15, 287 (1974) and Jarry et al., J. Poly. Sci., Poly. Phys, 18, 1879 (1980)) and in the processing of polymer fibers (Chappoy et al., Macromolecules, 12, 680 (1979)), but fluorescence techniques have not been employed to measure non-Newtonian viscosity of polymer melts and/or solutions undergoing shear flow.
On-line, real-time monitoring of the viscosity during the processing of polymer products is a necessary requirement for many manufacturers. Optimizing the viscosity in relation to other processing parameters can yield increased productivity and improved product performance. Knowledge of the molecular orientation can be used to predict and control the anisotropic character of the mechanical and electrical properties of the final product.
Current methods for measuring viscosity involve intrusive probes which disrupt the material flow or involve off-line units to which material is pumped from the main line. One advantage of the technique according to the present invention is that nonintrusive fluorescent measurements of viscosity can be carried out on-line.